Computed tomography method

ABSTRACT

The invention relates to a computed tomography method in which a rebinning operation is carried out so as to form parallel fan beams whose rays traverse a plane, containing the axis of rotation, in equidistant puncture points. Reconstruction is performed for rays which extend perpendicularly to said planes. Such a transition from a cone beam geometry to a parallel beam geometry enables very fast reconstruction which can be carried out notably for CT fluoroscopy.

BACKGROUND

The invention relates to a computed tomography method in which anexamination zone is irradiated by means of a conical radiation beam andin which the radiation source which generates the radiation beam rotatesalong a circular trajectory around an axis of rotation relative to theexamination zone. The invention also relates to a computed tomographyapparatus for carrying out the method as well as a computer program forcontrolling the computed tomography apparatus.

A method of this kind and a corresponding computed tomography apparatusare known from U.S. Pat. No. 6,285,733. The measuring values which areacquired therein by a two-dimensional detector unit are dependent on theintensity in the radiation beam to the other side of the examinationzone and are first subjected to a rebinning operation. This operationyields groups of measuring values which are associated with fan beamswhich are situated in equidistant fan beam planes which extend parallelto one another and to the axis of rotation. After the rebinningoperation, the fan beans are composed of rays which traverse a plane,containing the axis of rotation and extending perpendicularly to the fanbeam planes of the relevant group, in puncture points which are situatedon equidistant connecting lines which extend perpendicularly to the axisof rotation and parallel to one another.

The measuring data produced by the rebinning operation are subsequentlysubjected to one-dimensional high-pass filtering as well as tobackprojection in order to form at least one CT image. Thereconstruction of a voxel in the examination zone then takes intoaccount from each group the measuring data of rays having traversed therelevant voxel from different directions. If the voxel is not situatedin the central plane defined by the circular trajectory, the raystraverse the voxel at an angle relative to the central plane and eachray traverses the plane associated with its group at an angle other than90°. The reconstruction taking into account this cone beam geometry iscomparatively complex and hence requires a comparatively large amount ofcalculation time.

For various applications, however, shorter calculation times aredesired. This is the case, for example, in CT-guided biopsy (CT=computedtomography), where a biopsy needle is introduced into an object to beexamined and the advancing of the biopsy needle is continuously checkedon the basis of a series of three-dimensional CT images. In suchso-called CT fluoroscopy only very little time is available for thereconstruction of a CT image, notably when use is made of a so-called“sliding-window” technique in which a CT image is updated (while takinginto account newly acquired CT data and CT data already used for thereconstruction of the previous CT image) within a period of time whichis significantly shorter than the period of time required for theacquisition of the measuring data of a complete CT image.

SUMMARY

Therefore, it is an object to conceive a method of the kind set forth insuch a manner that the time required for the reconstruction of a CTimage is reduced. This object is achieved by means of a computedtomography method which includes the steps of:

-   a) generating, while using a radiation source, a conical radiation    beam which traverses an examination zone or an object present    therein,-   b) generating a circular relative motion, including a rotation about    an axis of rotation, between the radiation source on the one side    and the examination zone or the object on the other side,-   c) acquiring, while using a detector unit, measuring values which    are dependent on the intensity in the radiation beam to the other    side of the examination zone during the relative motion,-   d) rebinning the measuring values so as to form a number of groups,    each group containing the measuring values of fan beams which are    situated in equidistant fan beam planes which extend parallel to one    another and to the axis of rotation and are composed of rays which    traverse a plane which contains the axis of rotation and extends    perpendicularly to the fan beam planes of this group in puncture    points which are situated on equidistant connecting lines which    extend perpendicularly to the axis of rotation and parallel to one    another,-   e) reconstructing the spatial distribution of the attenuation of the    X-rays from the measuring data, formed by the rebinning of the    measuring values, for rays which extend perpendicularly to the    planes of the groups and through the puncture points so as to form    at least one CT image.

Whereas the reconstruction according to the known method takes placewhile taking into account the cone beam geometry prevailing during theacquisition of the measuring values, the present reconstruction is basedon a parallel beam geometry in which all beams extend perpendicularly tothe plane of the associated group and hence parallel to the centralplane defined by the circular trajectory. The amount of calculation workrequired for such a parallel beam geometry is substantially smaller thanthat required for a cone beam geometry. A further advantage resides inthe fact that in accordance with the invention a cylindrical part of theexamination zone is reconstructed.

Because the cone angle of the rays (being the angle enclosed by the raysrelative to a plane perpendicular to the axis of rotation) is de factoignored, the examination zone outside the central plane cannot beexactly reconstructed; this also holds for the known method, be it fordifferent reasons. The loss of image quality, however, is small if thedetector unit comprises only few detector lines or if the maximum coneangle is small. The image quality is notably higher than in the case ofa method in which the measuring values are treated from the very startas values from parallel planes in which each time one detector line issituated.

The claims 2 and 3 define proven methods for the reconstruction of a CTimage in the case of a parallel beam geometry. Such methods per seenable exact reconstruction of the examination zone, be it that thereconstruction is exact only within the central plane, because theparallel beam geometry on which the reconstruction is based occurs onlyin the central plane during the acquisition of the measuring values.

Claim 4 describes a preferred CT fluoroscopy application for fastreconstruction of the CT images, notably if the distances in timebetween two updates of a CT image are shorter than the period of timefor complete acquisition of the measuring values for a CT image.

Claim 5 describes a computed tomography apparatus for carrying out themethod and claim 6 describes a computer program for controlling acomputed tomography apparatus of this kind.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating the preferred embodiments and arenot to be construed as limiting the invention.

FIG. 1 shows a computed tomography apparatus which is suitable forcarrying out the invention,

FIG. 2 shows a flow chart of the method in accordance with theinvention,

FIG. 3 shows a conical radiation beam generated in a radiation sourceposition,

FIG. 4 shows the fan beams formed by the rebinning in parallel planes,

FIG. 5 is a cross-sectional view of these fan beams, and

FIG. 6 shows the cone beam geometry and the parallel beam geometry.

DETAILED DESCRIPTION

The computed tomography apparatus shown in FIG. 1 includes a gantry 1which is capable of rotation about an axis of rotation 14 which extendsparallel to the z direction of the co-ordinate system shown in FIG. 1.To this end, a motor 2 drives the gantry at a preferably constant butadjustable angular speed. A radiation source S, for example, an X-raysource, is connected to the gantry. The source is provided with acollimator device 3 which forms a conical radiation beam 4 from theradiation produced by the radiation source S, that is a radiation beamwhich has a finite dimension other than zero in the z direction as wellas in a direction perpendicular thereto (that is, in a planeperpendicular to the axis of rotation).

The radiation beam traverses an examination zone 13 in which an objectto be examined, for example, a patient on a patient table (both notbeing shown), may be situated. The examination zone 13 has the shape ofa cylinder. After having traversed the examination zone 13, the X-raybeam 4 is incident on a two-dimensional detector unit 16 which isconnected to the gantry 1. The detector unit comprises a number ofdetector lines which are situated adjacent one another in the zdirection and each of which comprises a plurality of detector elements.The detector lines are situated in planes perpendicular to the axis ofrotation and on an arc of circle around the radiation source S; however,they may alternatively describe an arc of circle around the axis ofrotation 14 or be straight. Each detector element provides a measuringvalue for a ray of the radiation beam 4 in each position of theradiation source.

The angle of aperture of the radiation beam, denoted by the referenceα_(max) (the angle of aperture is defined as the angle enclosed by aray, situated at the edge of the radiation beam 4 in a planeperpendicular to the axis of rotation, relative to the plane of thecentral ray defined by the radiation source S and the axis of rotation14), determines the diameter of the cylinder within which the object tobe examined is situated during the acquisition of the measuring values.The examination zone 13, or the object to be examined or the patienttable, can be displaced parallel to the axis of rotation 14 by means ofa motor 5. When the motors 5 and 2 are simultaneously activated, theradiation source S and the detector unit 16 perform a helical motionaround the examination zone 13. However, when the motor 5 for thetransport in the z direction is stationary and the motor 2 rotates thegantry separately, the radiation source S and the detector unit 16perform a circular scanning motion relative to the examination zone 13.

The measuring data acquired by the detector unit is applied to an imageprocessing computer 10 which reconstruct therefrom the absorptiondistribution in a part of the examination zone 13 and, for example,displays it on a monitor. The two motors 2 and 5, the image processingcomputer 10, the radiation source S and the transfer of the measuringvalues from the detector 16 to the image processing computer 10 arecontrolled by way of a suitable control unit 7.

FIG. 2 illustrates the execution of an acquisition and reconstructionmethod which can be carried out by means of the computed tomographyapparatus shown in FIG. 1.

After the initialization in block 101, the gantry rotates at a constantspeed, the duration of one revolution being 1 second or less. Theradiation source S emits a conical radiation beam which traverses theexamination zone and the measuring values acquired by the detectorelements of the detector unit 16 are buffered in the image processingcomputer 10 so as to be further processed.

FIG. 3 shows the circular trajectory described around the axis ofrotation 14 by the radiation source S and the detector unit 16. Thetrajectory 17 is situated in a plane which is perpendicular to the axisof rotation and which will be referred to hereinafter as the centralplane. The Figure shows a radiation beam 4 which is emitted by theradiation source in a given radiation source position S₀. This conicalradiation beam can be decomposed into a plurality of flat fan beamswhich are situated, like the fan beams 401, 402, 403 shown in FIG. 3, inplanes extending parallel to the axis of rotation 14. The fan beamsemanate from the same radiation source position and are detected by arespective column of detector elements on the detector unit 16, saidcolumn extending parallel to the axis of rotation 14.

FIG. 3 indicates that the emitted conical radiation beam is alsomeasured in other positions of the radiation source, for example, S⁻²,S⁻¹, S₁ or S₂. These radiation source positions, or the radiation beamsemitted therein, can be characterized by a parameter β which correspondsto the angle enclosed by the normal from the radiation source positionto the axis of rotation 14 relative to a reference line in the centralplane (β may be larger than 2π in conformity with the number ofrevolutions of the radiation source around the axis of rotation). Theposition of each fan beam in a radiation beam can be characterized by aparameter s which describes the position of the column of detectorelements, being struck by the fan beam, within the detector unit 16.Each ray within such a fan beam itself can be characterized by theparameter t which describes the position of the detector element, beingstruck by the relevant ray, within the column of detector elements, orthe distance between this detector element and the central plane.

The acquired measuring values form in this manner a three-dimensionaldata set M₀(β, s, t), each measuring value corresponding to a grid pointof a regular Cartesian grid in a three-dimensional (β, s, t) parameterspace. The acquisition of the measuring values thus constitutes asampling of the so-called object function (in this case of the lineintegral of the attenuation of the radiation) at a number of pointswhich are regularly distributed in the (β, s, t) parameter space.

The acquisition of the measuring values in the step 102 and theprocessing of these measuring values in the subsequent steps 103 andfurther take place in parallel in time, so that the acquired measuringvalues can already be further processed while further measuring valuesare still being acquired.

In the step 103 the measuring values are multiplied by the cosine of theangle enclosed by the ray, along which the measuring value was acquired,relative to a plane perpendicular to the axis of rotation. If thedimensions of the detector unit are small in the z direction, however,this step can be dispensed with, because in that case the angle is sosmall that the cosine of the angle is practically always 1.

The data set M₀(β, s, t), acquired in the step 102 and possibly modifiedin the step 103, is not yet optimum for further processing. Therefore,in the steps 104 and 105 a so-called rebinning of the measuring valuesis carried out. The data is then resorted and re-interpolated as if ithad been measured with a different radiation source (a circularradiation source emitting mutually parallel fan beams) and with adifferent detector (a flat, rectangular detector containing the axis ofrotation). To this end, in the step 104 first the fan beams which aresituated in planes which are parallel to one another and to the axis ofrotation 14 and emanate from different radiation source positions arecombined so as to form respective groups.

FIG. 4 shows a group of fan beams formed in this manner. Each time onefan beam of each radiation source position S⁻². . . S₀ . . . S₂ belongsto such a group. The fan beams associated with a group satisfy thecondition:φ=α+β  (1)Therein, φ is the projection direction in which a group of fan beamsextends through the examination zone. α is the angle enclosed by therelevant fan beam in the original radiation beam (see FIG. 3) relativeto a plane which is defined by the axis of rotation 14 and the radiationsource position (for example, S₂ which itself is defined by the angleβ). Groups of fan beams of this kind are formed for different projectiondirections φ which deviate from one another each time by a givenprojection angle increment dφ. When the fan beams of a radiation beam donot exactly satisfy the equation (1), a corresponding fan beam must bedetermined by interpolation from the rays of fan beams neighboring thefan beam 4 (FIG. 3).

The fan beams of a group, including the fan beams 411 . . . 415 shown inFIG. 4, define a radiation beam 410 which has a tent-like shape and iscomposed of fan beams which are situated in planes which extend parallelto one another and parallel to the axis of rotation. FIG. 4 also showsthe area of intersection 420 which is obtained when the radiation beam410 is intersected by a plane which contains the axis of rotation 14 andextends perpendicularly to the planes of the fan beams 411 . . . 415.The upper and lower edges are curved outwards in the form of a cushion,because the radiation source positions at the center are situatedfurther from the plane of intersection than the radiation sourcepositions at the edge.

FIG. 5 is a detailed representation of the area of intersection 420.Round dots mark the positions in which the rays of the fan beams 411 . .. 415 puncture the area of intersection. For the outer fan beams 411 and415 these dots are situated nearer to one another than for the inner fanbeams 412 . . . 414, and in one of the inner fan beams (for example,413) the distance between these points decreases in the direction fromthe center towards the edge. The area of intersection 420 contains arectangular area 160 whose upper edge 161 and lower edge 162 are givenby the dimensions of the two outer fan beams 411 and 415 in the area ofintersection 420 and which will also be referred to as “virtualdetector” hereinafter.

In the step 105 the rays of the fan beams 411 . . . 415 of a group arecalculated again by interpolation, that is, in such a manner that theytraverse the virtual detector 160 in puncture points which are situatedon mutually parallel, equidistant connecting lines which extendperpendicularly to the axis of rotation 14. The distance between thepuncture points on each connecting line is constant. The puncture pointsresulting from this rebinning step are marked by a symbol “+” in FIG. 5and one of the connecting lines extending parallel to the edges 161 and162 is represented by the dashed line G in FIG. 5.

The two rebinning steps 104 and 105 transform in this manner themeasuring values M₀(β, s, t), defined by a regular grid in the (β, s, t)parameter space, into measuring values M₁(φ, u, v) which are defined bya regular grid in a three-dimensional (φ, u, v) parameter space. Theparameters u and v represent the co-ordinates of the puncture points inthe direction perpendicular to and parallel to the axis of rotation,respectively, in the virtual detector 160. The values M₁(φ, u, v)produced by the rebinning operation will be referred to as measuringdata hereinafter so as to distinguish them from the measuring valuesM₀(β, s, t) produced by the measurement.

Because the rays extending through the area of intersection 420 abovethe upper edge 161 and below the lower edge 162 are not used for theremaining part of the method, it is advantageous to configure thecollimator arrangement 3 (FIG. 1) in such a manner that the conicalradiation beam does not contain these rays. Instead of straight edgesextending perpendicularly to the axis of rotation, the collimatorarrangement 3 should have edges curved inwards for this purpose. Theradiation load for the patient would thus be reduced.

The invention as described so far with reference to FIG. 2 is known fromU.S. Pat. No. 6,285,733. In accordance with the invention, however, thefurther processing of the measuring data is not carried out on the basisof a cone beam geometry but on the basis of a parallel beam geometry.This difference in processing is symbolically represented by the dashedbox 106 and illustrated in FIG. 6. The solid lines in FIG. 6 representthe diverging rays of a fan beam (for example, 413) emanating from aradiation source position (for example, S₀). These rays traverse thevirtual detector 160 in equidistant puncture points. The invention,however, instead utilizes the rays which are represented by dashed linesand are incident perpendicularly on the virtual detector 160. Themeasuring data produced by the rebinning operation are associated withthe parallel rays traversing each time the same puncture point of thevirtual detector 160 as the diverging rays emanating from the radiationsource position S₀.

This transition to parallel rays does not require a separate calculationstep (which is why the box 106 is denoted only by dashed lines), becauseit is merely necessary to ignore the cone angle (being the angleenclosed by the rays relative to a plane perpendicular to the axis ofrotation 14). A multi-slice method, in which the data is acquired inparallel slices, then enables reconstruction by taking into account themeasuring data of rays which are situated each time in the same plane,perpendicular to the axis of rotation 14, so as to reconstruct theslices independently of one another.

The invention thus enables the reconstruction of a cylindrical part ofthe examination zone whose height corresponds to the distance betweenthe two edges 161 and 162 of the virtual window 160. The height h of thecylindrical part of the examination zone is in conformity with therelation:h=r tan(γ_(max))cos(α)  (1)Therein, r is the radius of the circular trajectory 17, γ_(max) is themaximum cone angle and α is the fan angle, that is, the angle ofaperture of the radiation beam in a plane perpendicular to the axis ofrotation.

In the geometrical conditions described above, a reconstruction isperformed by a filtered backprojection in the steps 107 to 109. In thestep 107 a one-dimensional ramp-like filtering of the measuring dataM₁(θ, u, v) is carried out. All measuring data having the sameprojection angle θ and the same parameter v are then subjected to afiltering operation during which the transfer factor increases ramp-likeas a function of the frequency.

The data F(θ, u, v) thus filtered is then subjected to backprojection.In the step 108 a voxel P(x, y, z) is defined. In the subsequent step109 the contributions to the attenuation value of the relevant voxel bymeasuring data whose associated rays traverse the voxel from differentprojection directions φ and are situated in the same plane parallel tothe central plane as the voxel are determined and summed. This isrepeated for all voxels in said plane, so that the object function forthis plane is known. The steps 108, 109 are carried out for all planesparallel to the central plane and the voxels and rays present in theserespective planes. Subsequently, the CT image reconstructed in thismanner can be displayed (step 110).

In the case of a fluoroscopic computed tomography method the steps 101to 110 form part of a continuous acquisition and reconstruction processduring which continuously new measuring values and new CT images of theexamination zone are formed. The distance in time between updates of aCT image can then be shorter than the period of time required by thecomputed tomography apparatus so as to acquire measuring values for acomplete CT image. In this case the reconstruction of the CT image takesplace partly from measuring values which are acquired anew and partlyfrom measuring values which have already been used for thereconstruction of the preceding CT image.

Instead of using a filtered backprojection in the steps 107 to 109, thespatial distribution of the attenuation in the planes defined by thepuncture points can also be determined by means of an inverse Fouriertransformation after the measuring data have first been transformed on aCartesian grid in the individual planes by means of a gridding method.

The invention has been described with reference to the preferredembodiments. Modifications and alterations may occur to others uponreading understanding the preceding detailed description. It is intendedthat the invention be constructed as including all such modificationsand alterations insofar as they come within the scope of the appendedclaims or the equivalents thereof.

1. A computed tomography method which includes the steps of: a)generating a conical radiation beam which traverses an examination zoneor an object present therein, b) rotating the conical radiation beamrelative to the examination zone or the object about an axis ofrotation, c) acquiring measuring values which are dependent on theintensity in the radiation beam on the other side of the examinationzone during the relative rotation, d) rebinning the measuring values soas to form a number of groups, each group containing the measuringvalues of fan beams which are situated in equidistant fan beam planeswhich extend parallel to one another and to the axis of rotation and arecomposed of rays which traverse a plane which contains the axis ofrotation and extends perpendicularly to the fan beam planes of thisgroup in puncture points which are situated on equidistant connectinglines which extend perpendicularly to the axis of rotation and parallelto one another, e) reconstructing the measuring values representing thefan beams of rays in the fan beam planes by treating the measuringvalues as representing rays which extend parallel to each other andperpendicularly to the transverse plane through which the puncturepoints are defined to form at least one CT image.
 2. A computedtomography method as claimed in claim 1, in which the reconstructionstep comprises: a) one-dimensional filtering of the measuring data,formed by the rebinning operation, of each group in the direction of theconnecting line, b) backprojecting the filtered data of a plurality ofgroups.
 3. A computed tomography method as claimed in claim 1, in whichthe reconstruction step includes an inverse Fourier transformation. 4.The method claimed in claim 1 wherein continuous acquisition ofmeasuring values for further CT images takes place while CT images arecontinuously being reconstructed.
 5. A computed tomography apparatus,which apparatus includes: a) a radiation source for generating a conicalradiation beam which traverses an examination zone or an object presenttherein, b) a drive device for realizing a circular relative motion,including a rotation about an axis of rotation, between the radiationsource and the examination zone or the object, c) a detector unit forthe acquisition of measuring values, during the relative motion, whichmeasuring values are dependent on the intensity in the radiation beam tothe other side of the examination zone, and also includes an imageprocessing unit for generating at least one CT image from the measuringvalues by performing the steps of: d) rebinning the measuring values soas to form a number of groups, each group containing the measuringvalues of fan beams which are situated in fan beam planes which extendparallel to one another and to the axis of rotation, rays of the fanbeams traverse a plane which extends perpendicular to the fan beamplanes, e) reconstructing a spatial distribution of an attenuation ofthe radiation from the measuring values, by treating the fan beam raysof the parallel fan beam planes of the groups as parallel rays whilereconstructing at least one CT image.
 6. A computer readable mediacomprising a program for controlling a computed tomography apparatus toperform the steps of: a) causing a radiation source to generate aconical radiation beam which traverses an examination zone or an objectpresent therein, b) causing rotation of the radiation source relative tothe examination zone or the object about an axis of rotation, c) causinga detector unit to acquire measuring values which are dependent on theintensity in the radiation beam to an opposite side of the examinationzone from the radiation source during the relative motion, d) rebinningthe measuring values so as to form a number of groups, each groupcontaining the measuring values, each group corresponding to fan beamssituated in equidistant fan beam planes which extend parallel to oneanother, the fan beam planes of each group being perpendicular to agroup plane which contains the axis of rotation, e) reconstructing aspatial distribution of an attenuation of the radiation from themeasuring values by treating the measuring values as representing raysextending perpendicularly to the group plane of the corresponding groupto form a continuously updated fluoroscopic CT image.